Field of the Invention
The present invention relates to carbon molecular sieve membranes and gas separations utilizing the same.
Related Art
Membranes are often preferred to other gas separation techniques in industry due to the following advantages. The energy consumption for membranes is low as they do not require a phase change for separation. Membrane modules are compact, thereby reducing their footprint and capital cost. Membranes are also mechanically robust and reliable because they have no moving parts.
Polymer membranes in particular are used in a wide variety of industrial applications. They enable the production of enriched nitrogen from air. They separate hydrogen from other gases in refineries. They are also used to remove carbon dioxide from natural gas.
However, owing to the manufacturing processes and material structure, today's polymeric membranes cannot reach both high selectivities and permeabilities, because a trade-off exists between permeability and selectivity. Robeson formulated semi-empirical upper-bound trade-off lines for several gas pairs. (Robeson, “The upper bound revisited”, Journal of Membrane Science 2008, vol 320, pp 390-400 (2008)). Carbon membranes exceed this upper-bound and therefore are quite promising.
Carbon molecular sieve membranes (CMS membranes) may be obtained by high-temperature pyrolysis under oxygen-deficient atmospheres of polymer precursors. They offer attractive gas separation properties relative to the precursor polymer mainly characterized by higher permeance, higher selectivity, a tolerance to higher process gas temperatures, and a resistance to plasticization-induced selectivity losses due to condensable components in gas feeds. That is, CMS membranes bypass the traditional tradeoffs between permeability and selectivity that polymer membranes suffer from, yielding simultaneously higher productivity and higher product purity. To further improve the module productivity per unit volume, the membranes can be produced in a hollow-fiber geometry. Together with plasticization resistance, these properties render CMS hollow-fiber membranes ideal candidates for many separations, including natural gas upgrading and olefin/paraffin separations.
The gas transport properties of the CMS membrane are strongly dependent upon those of the polymer precursor. Glassy polyimides are particularly attractive precursors partly because of their thermochemical properties: high decomposition temperatures, high carbon content, and high glass transition temperatures (Tg). It has been shown that the use of a high-free-volume precursor polymer with a high Tg (e.g., 6FDA/BPDA-DAM) results in a CMS membrane with a higher permeance than that derived from a low-free-volume precursor polymer with a lower Tg (e.g., Matrimid). In this particular example, the higher Tg is thought to help prevent collapse of the polymer structure during temperature ramping in pyrolysis and thus to promote the maintenance of the high free volume and permeance in the CMS membrane.
However, one of the primary drawbacks hindering the commercialization of CMS membranes is the severe “aging” behavior they undergo, shown by prominent reductions in gas permeance with time. This impedes the design of economical processes around the CMS membranes. A body of literature suggests that the aging is primarily “physical” in nature, being due to an in-situ, intrinsic “collapse” of the thin separating layer. Contributions of “chemical aging” due to sorption of ambient oxygen or other molecules have been minimized by demonstration of aging behavior under vacuum. With Struik's theories on aging in mind, the pyrolysis-induced mass loss results in a higher surface area and associated higher free volume in the CMS membrane in comparison to the precursor membrane. This higher free volume dissipates naturally over time as a denser “equilibrium” packing is reached. This phenomenon is illustrated in FIG. 1. After pyrolysis of the polyimide of the green fiber to produce the CMS membrane fiber, the spacing in between polymer chains begins at a first value, say, δ1. Over time, the physical-aging densification of the thin separating layer causes that spacing to decrease to a second value, say, δ2, where δ1>>δ2. This is known to occur more rapidly in the relatively thinner (˜100 nm) separating layers that are characteristic of industrial gas separation membranes. Therefore, high permeance CMS membranes derived from high-free-volume precursors like 6FDA/BPDA-DAM made according to conventional techniques do not appear to retain their advantage in permeance upon aging. Indeed, there can sometimes be an orders of magnitude decrease after aging. The aging effect can generally be observed within one week of formation of the CMS membrane fibers.
Metals have been added into carbon molecular sieve membranes for a variety of purposes.
U.S. Pat. No. 7,947,114 discloses production of CMS membranes (made by pyrolyzing cast films of cellulosic materials and metal salts for purposes of performing electroregeneration of the CMS membranes, whereby the conductivity of the membrane is improved so that an electric current can be used to heat the membrane and assist in removal of adsorbed contaminants.
Lie, et al. discloses production of CMS membranes (made by pyrolyzing cast films of cellulosic materials and metal salts. Lie, et al., Carbon membranes from cellulose and metal loaded cellulose, Carbon 43 (2005) 2600-2607. Each of the CMS membranes incorporating a metal salt exhibited significant aging effect while comparative CMS membranes without metal salts did not exhibit aging.
Barsema, et al. discloses production of silver nanocluster-containing CMS membranes (made by pyrolyzing cast films of polymer and either AgNO3 or CH3COOAg), partly for the purpose of achieving facilitated transport of a gas. Barsema, et al., Functionalized carbon molecular sieve membranes containing Ag-nanoclusters, Journal of Membrane Science 219 (2003) 47-57. The aging effect was not studied.
Therefore it is an object of the invention to provide a CMS membrane (and method of making the same) that does not experience the aging effect exhibited by conventional CMS membranes and thus retain their advantage in permeance.